Sea ice conditions in the Arctic Northeast Passage (NEP) have changed dramatically in the last four decades, with important impacts on the environment and navigability. In the present study, multisource remote sensing data for 1979-2012 were analyzed to quantify seasonal, interannual, and spatial changes in sea ice conditions along the NEP. Data for October-November showed that spatially averaged ice thickness in the NEP decreased from 1.2-1.3. m in 2003-2006 to 0.2-0.6 m in 2011-2012. From 1979 to 2012, the fastest decreasing trend in monthly ice concentration occurred in October (-1.76% per year, P< 0.001), when the ice cover starts to increase. As a result of decreasing multiyear sea ice, thinning ice and delayed freeze-up, the spatially averaged length of open period (ice concentration < 50%) increased from 84. days in the 1980s to 114. days in the 2000s and reached 146 days in 2012. The Kara, Laptev, and East Siberian sectors were relatively inaccessible, especially the sector of 90-110°E around the Vilkitsky Strait. However, because of the thinning sea ice prior to the melt season and the enhanced positive polarity of the summer Arctic Dipole Anomaly, these sectors have become more accessible in recent years. The summer sea ice along the high-latitude sea route (HSR) north of the eastern Arctic islands, with a route distance comparable to the NEP, has also decreased during the last decade with the ice-free period reaching 42. days in 2012. The HSR avoids shallow waters along the coast, improving access to the Arctic sea route for deeper-draft vessels.